The increase in the traffic and in the size of the contents transported in satellite communication networks is giving rise to an increase in the loading on these networks.
To cope with this increase in loading, it is necessary to implement efficient techniques for allocating resources within satellite networks, so as to increase the capacity of these networks, and also to afford them flexibility, by allocating the resources that they have to the locations where they are most required.
In order to increase the overall capacity of a satellite communications network, it is conventional to cut the geographical zone covered by the network into a plurality of sub-zones of smaller sizes, called satellite spots, illuminated by a beam of the satellite. The union of the set of these spots forms the geographical coverage zone, in a multibeam satellite system.
Cutting into beams makes it possible to improve the general capacity of the satellite network. Indeed, the antennas covering each of the beams being more directional, their gain, and therefore the link budget, is more favourable, thereby making it possible to use modulation and coding schemes of greater spectral efficiency. Moreover, when the spatial isolation between the spots, in terms of antenna gain, is sufficient, the same spectral resource can be used several times within the satellite network.
In order that the various beams not interfere with one another, the closest beams use distinct frequencies, the more distant beams being able to use like frequencies.
The general capacity of the satellite system is therefore a product of the capacity of each beam and of the total number of beams, the number of beams being limited by the beamforming capacity of the antennas, the payload carrying capacity and the interference between beams, which vary as a function of the antenna patterns, of the frequency reuse scheme and of the size of the spots.
Recently, massively multibeam system architectures have been proposed to allow a communications satellite to attain capacities of greater than 150 Gbps. These are systems relying on the use of the Ka band, at least as regards the user half-link (that is to say the link between the gateway and the user terminal via the satellite). The obtaining of such a capacity relies on optimized payload architectures that are very efficient in terms of capacity per unit mass and per unit power.
FIG. 1 illustrates the result of a method of reusing resources according to the prior art, in which various spectral resources are allotted, or allocated, to various beams and reused so as to minimize the interference between the beams using the same bands. In the example, the reuse is done using a scheme with N=4 colours.
In FIG. 1, a geographical zone 110 is cut up into multiple beams 111 to 144. In the example, the total frequency resource allotted to the whole network is divided into four bands of frequencies: f1, f2, f3 and f4.
A band of frequencies is allotted to each of the beams, so as to maximize the gap between two beams using the same frequency resource. In the example, the frequency reuse scheme equals N=4, one then speaks of a reuse scheme with N colours. In this reuse scheme, one and the same frequency resource is never allotted to two adjacent beams, but can be reused by bi-adjacent beams. This is the case for example for the beams 112, 114, 131 and 133. The frequency resources f1, f2, f3 and f4 can each correspond to a quarter of the total band allotted to the network. Another implementation consists in allocating half the total band to the frequencies f1 and f2, and in using the same bands of frequencies as f1 and f2 in the orthogonal polarization for the bands of frequencies f3 and f4.
The presence of zones 151 of intersections between two satellite beams will be noted in FIG. 1. In these intersection zones, the frequency resources of several beams are received with comparable power levels.
Most of the time, the same quantity of spectral resources is allotted to each of the beams. They do not then make it possible to offer non-uniform capacity distribution over the service zone. However, analyses of the demand for capacity for future systems reveal wide geographical disparities, whose distribution may vary over time as a function of the evolution scenario of the market considered. The relevant metric in this situation is then no longer the overall capacity of the system, but the capacity from which value can actually be gleaned.
In order to cope with this disparity in demand over the whole of the network, it is therefore desirable to afford flexibility to the distribution of capacity. This requirement for flexibility pertains by priority to the outbound pathway (that is to say the link from the gateway to the user terminal via the satellite).
One way of satisfying the local spikes in demand for capacity is to increase the capacity of the whole set of beams of the satellite system. For this purpose, techniques for so-called fractional frequency reuse (known by the acronym FFR) are known to the person skilled in the art.
The method of fractional frequency reuse consists in overlaying two frequency reuse schemes. A first frequency resource is allotted to each of the beams according to a frequency reuse scheme with N colours, as in the frequency reuse method illustrated in FIG. 1, and then a second frequency resource is allotted to each of the beams, according to a reuse scheme with P colours, P lying between 1 and N. The frequency resources associated with the two colour schemes are distinct. The transmissions in the various beams are then done by combining the use of the resources associated with the scheme with P colours and the use of the resources associated with the scheme with N colours.
In the conventional case, P equals 2, or 1 when polarization diversity is used. An embodiment represented in FIG. 2 using polarization diversity, and in which N=4 and P=2, consists in dividing the frequency band into three sub-bands f1, f2 and f5, the bands of frequencies associated with the scheme with N colours then being the sub-bands f1, f2, as well as the sub-bands f3 and f4, identical to f1 and f2 but in the orthogonal polarization. The bands of frequencies associated with the reuse scheme with P=2 colours then use the sub-band f5, and the sub-band f6 identical to f5 but in orthogonal polarization.
In each beam, the use of the frequency resource allotted according to the scheme with P colours is reserved for the terminals for which the interference generated by the simultaneous use of these resources within other beams to which they are allotted does not disturb the communications. The identification of the terminals that are eligible to use this frequency resource is generally done on the basis of the geographical position of the terminal, the terminals situated at the centre of the satellite spot being the most liable to be robust to interference, on account of the directivity of the antennas used for each beam.
These terminals can then use the first and second frequency resources allotted to the beam to which they belong. The other terminals, which are less robust to interference, use only the frequency resources associated with the frequency reuse scheme with N colours. The overall capacity of the network is then increased, through the use of the second frequency resources.
The increase in the capacity of the network is related to the size of the zone comprising the terminals that can use the second frequency resource. This size depends on the system's transmission parameters; it must represent an appreciable portion of the spot so as to increase the capacity for this beam, but be limited so as not to increase the quantity of interference generated between the beams.
FIG. 2 illustrates the implementation of such a fractional frequency reuse method, in which a first reuse scheme with N=4 colours is used jointly with a reuse scheme with P=2 colours. In this embodiment, the terminals of the beam 111 that are situated in the zone 211, but also the terminals situated at the centre of beams whose colour is identical to that of the beam 111 in the reuse scheme with P=2 colours, such as for example the beams 112 to 114 and 131 to 134, can use one and the same second frequency resource f5 whilst the beams 121 to 124 and 141 to 144 can use one and the same second frequency resource f6.
Though the implementation of the fractional reuse of frequencies on all the beams makes it possible to increase the capacity of the system, it gives rise to a noticeable further complicating of the payload, in particular the power section. It is then suitable only when the bandwidth requirement on the half-link between the satellite and the gateway increases significantly, and is not really a response that copes with the geographical disparities in the requirement for capacity on the various beams.
Other techniques known to the person skilled in the art allow a gain in terms of capacity distribution flexibility.
A first technique consists of the use of a method of frequency reuse with N colours, and the allotting of more or less power to the various beams as a function of their loading. Since increasing the power makes it possible to improve the link budgets, it is therefore possible to use modulation and coding schemes having better spectral efficiency for these beams. However, the gain afforded by such a technique is limited, the power increase for a beam also giving rise to an increase in the level of the interference with the beams using the same frequency resource.
A second technique consists of the use of a method of frequency reuse with N colours, and the dynamic allocating of the terminals between the various beams so as to distribute the loading from a loaded beam to a less loaded beam. Indeed, the terminals situated in zones of intersections between two beams, or at the periphery of a beam, can be allotted equally to one or the other of the beams, thus making it possible to balance the loading of the beams, and therefore to afford flexibility. However, this technique is only moderately efficient since it relates to only a limited number of terminals. The link budget decreases very quickly when the technique selects terminals receding further from the edges of the beam.
Thereafter, a third technique consists in using a method of frequency reuse with N colours, and in optimizing the width of the frequency band allocated to the beams as a function of the loading, so as to allot more band to the beams having the largest demand for capacity. Although efficient as regards flexibility, the implementation of this technique requires the use of RF (Radio Frequency) components that can be reconfigured dynamically on the satellite. These components, such as for example RF filters or else switches, cause an appreciable increase in the mass and/or in the power consumed by the payload onboard the satellite. Flexibility is therefore obtained as a counterpart to an increase in the payload onboard the satellite.
Finally, a fourth technique makes it possible to afford flexibility to the satellite network by allocating the resources to the various beams according to a so-called beam hopping method. This method defines a hop frame containing various timeslots, and carries out a temporal and frequency allocation of the resources to the various beams. Accordingly, groups of beams are constructed, which use one and the same frequency resource. The beams of the group are alternatively illuminated as a function of the allocation of the timeslots of the hop frame. By allocating more or fewer timeslots to the beams as a function of their loading, this method optimizes the use of the spectral resource,
FIG. 3a illustrates such operation, in the case where the channels are grouped together in pairs. In FIG. 3a, the loading must be distributed between a first beam 301 and a second beam 302. The principle can be extended in an identical manner to a larger number of beams and to non-adjacent beams. In the example, the beam 302 is overloaded, while the beam 301 is under-loaded. The allotting of the timeslots of the hop frame 311 between the two beams will then favour the beam 302. In the example, two slots out of eight are allotted to the beam 301, and six slots out of eight are allotted to the beam 302. These beams using the same frequency resource, the beam 302 will therefore have 75% of the resource, with 25% for the beam 301. This method makes it possible to adapt the distributing of the resources as a function of requirements, and therefore to afford flexibility to the network.
In the particular case where 50% of the timeslots are allotted to each of the slots, and where the spots have the entire frequency resource, the capacity of each of the beams is equal to that obtained by a resource reuse method for which N equals 2 (4 when polarization diversity is used).
The sequentially allocated frequency band can consist of one or more carriers. It is then the whole set of carriers that is switched from one beam to the other.
Such a technique therefore affords a local response to the requirements for flexibility of the loading between two adjacent beams. At each instant, a single beam can have the whole set of frequency resources, this temporal separation making it possible to ensure the absence of interference between the various beams which share the hop frame. The use of components such as for example ferrite switches, makes it possible to carry out the fast switching function aboard the satellite (typically every millisecond), for limited extra payload mass in the satellite.
However, this solution does not make it possible to cope with the problem of the increase in the loading in a geographical zone comprising several beams.
FIG. 3b illustrates this problem. To the beams of FIG. 3a are added the beams 303 and 304. The beam 303 is overloaded, and the beam 304 under-loaded. The increase in the loading of a satellite network usually being localized around a group of beams, this case is a commonplace practical case.
The use of a method of beam hopping then makes it possible to allot various timeslots to the beams 303 and 304 in the hop frame 312. In order to distribute the loading between the beams as a function of demand, five time slots are allotted to the beam 303 and three to the beam 304.
In this specific case, the beams 302 and 303, adjacent and lacking resources, both use the whole set of frequency resources in the timeslots 320. This use generates interference that reduces the capacity and the availability of the service at the edge of each of these beams, and will ultimately considerably degrade the overall capacity of the network.
The use of beam hopping therefore makes it possible to distribute the loading locally between adjacent beams, but does not solve the flexibility problem when the latter is envisaged over the whole network.
U.S. Pat. No. 6,377,561 B1 describes another way of satisfying the local spikes in demand for capacity, by grouping the beams into groups of adjacent beams, and by distributing the resources in frequency and in time inside each of these groups as a function of the loading. This technique affords flexibility at the level of a group of beams, but does not cope with the problem of flexibility when a high demand for capacity is localized on a group of adjacent beams, and does not solve the problems of interference between adjacent beams of groups of adjacent beams.
Patent application US 2014/0286236 A9 describes for its part a technique in which the allocations of frequencies to the beams are flexible over time and in space. However, this technique does not deal with the problems of interference between adjacent beams, and is therefore sub-optimal in terms of overall capacity.